1. Field of the Invention
The present invention concerns a solution containing a fluorescent whitening agent and, in particular, a solution of this type suited for use in the formulation of liquid detergents both for hand washing and for machine washing.
Liquid detergents for washing are, in recent years, increasingly conquering the market which was traditionally--essentially for technical reasons--mostly enjoyed by solid detergents, both in powder and in granules. At present, liquid detergents form 15-20% of the total amount of detergents produced in Europe, while reaching 40% of the total amount produced in North America.
The fluorescent whitening agents (hereinafter called FWA for the sake of brevity) are substances which act by absorbing the ultraviolet radiation and re-emitting it at longer wave lengths, corresponding to the blue/violet radiation in the visible field. When the FWA are applied to a fabric, mixed with a detergent solution, they eliminate or reduce any undesirable yellowish shading on white fabrics and they give a brighter and more brilliant hue to coloured fabrics. The known FWA belong to various classes of different chemical compounds, but the ones that are by far the most widely known and used in detergents, are the stilben-triazine-based FWA, which are particularly suited to develop their properties on prevalently cellulosic substrata.
2. Description of the Prior Art
In producing liquid detergents, it is evidently preferable for all the basic components forming part of the formulation to be available in a liquid state from the outset. This allows simplifying the plant for producing the detergents, as well as the automation of the different operations for proportioning and mixing the individual components.
The preferred condition hardly applies to the commercially known FWA, which are scarcely water soluble and are thus used only in solid form to produce liquid detergent compositions. Such FWA are included in the general formula: ##STR2## wherein X is sodium, potassium, ammonium or an amine; R and R' are the phenyl group, or the phenyl group substituted with methyl, ethyl, or halogens.
Among the different FWA corresponding to the above general formula, the one wherein R=R'=phenyl is particularly interesting, both because it has already been used in the sector and because it has the property to prevent the forming of bleaching stains (non-staining effect), as described for example in EP-A-237 119. The present invention thus specifically refers to the use of an FWA having such a structure, simply indicated hereinafter as "FWA of the invention".
Given the low solubility of the FWA of the invention, in present processes for producing liquid detergents--which make use of said FWA--a specific step has to be provided for dissolving the FWA in said detergent or, alternatively, to pre-dissolve the FWA in the surface-active base of the detergents. This requires a more complicated plant, supplementary times for mixing and dissolving the FWA and, ultimately, an increase in the final cost of the detergent. It would hence be important and extremely useful--in order to facilitate and make more economical the preparation of liquid detergents containing said type of FWA--to be able to use the FWA of the invention as a clear fluid solution, easy to be metered.